My wife tells me this morning...with about 2" of ice on her car and not a bottle of de-icer in sight, that her car (Fiesta) heater hasn't worked for a few weeks.
To Start with, is there any coolant in the System & do you have antifreeze in it?
With the Engine running go under the Bonnet & feel the heater pipes, are they Hot?
If yes, Air Lock in the System,
Does the Heater with the Controls in the Hot Position, go Hot / Cold /Hot / Cold?
If yes that indicates an Air Lock.
Try the Thermostat if all else fails.